With the advances in technology, projectors are getting more and more popular due to having an improved performance, smaller size and lower price. Consequently, more and more patents related to projectors are disclosed; for example, U.S. Patents (U.S. Pat. No. 6,371,617 and U.S. Pat. No. 7,090,357) and Taiwan Patent (patent No. 1300834) disclose the structures of the projectors.
Please refer to FIGS. 1 and 2 both. The conventional projector 100 includes a light source 110, a light integration rod 120, a total internal reflection prism (TIR prism) 130, a digital micro-mirror device (DMD) 140, and a projection lens 150. The light source 110 is configured to provide an illumination beam 112. The light integration rod 120 is configured to receive the illumination beam 112 and uniformly output the illumination beam 112 to the total internal reflection prism 130 and has a light incidence surface 121 and a light emission surface 122. Each of the light incidence surface 121 and the light emission surface 122 is in a rectangular shape. Specifically, an area of the light emission surface 122 is greater than an area of the light incidence surface 121; the long edge 123 of the light incidence surface 121 and the long edge 124 of the light emission surface 122 are parallel to each other; and the short edge 125 of the light incidence surface 121 and the short edge 126 of the light emission surface 122 are parallel to each other.
The total internal reflection prism 130 has a total reflection surface 132 configured to reflect the illumination beam 112 to the digital micro-mirror device 140. The digital micro-mirror device 140 is configured to convert the illumination beam 112 into an image beam 113. The incidence angle of the image beam 113 relative to the total reflection surface 132 is configured to be smaller than a critical angle, so the image beam 113 passes through the total reflection surface 132 and then further emits into the projection lens 150. The projection lens 150 is configured to project the image beam 113 onto a screen (not shown) thereby forming an image on the screen.
The digital micro-mirror device 140 includes an active area 142 and a protective film 144 covering the active area 142. The active area 142 includes a plurality of micro-mirrors (not shown), and each of the micro-mirrors rotates within +12 degrees and −12 degrees. Specifically, the micro-mirror is operated in an on-state when rotating to +12 degrees, and the on-state micro-mirror is configured to reflect lights into the projection lens 150; alternatively, the micro-mirror is operated in an off-state when rotating to −12 degrees, and the off-state micro-mirror is configured not to reflect lights into the projection lens 150. That is, the light reflected into the projection lens 150 from the on-state micro-mirrors is also referred to as the aforementioned image beam 113. In addition, besides being reflected by the active area 142, a portion of the illumination beam 112 is reflected by the protective film 144.
Please refer to FIGS. 1 and 3 both. The light spot formed by the image beam 113 is supposed to be in a circle-like shape; however, since a portion of the image beam 113 having a relatively large incidence angle may not successfully pass through the total reflection surface 132 when the image beam 113 emits the total reflection surface 132 of the total internal reflection prism 130 and thereby resulting in light loss, a light-loss area resulted from the total internal reflection prism 130 is occurred on the right side of the light spot S1 formed in the diaphragm of the projection lens 150 consequently.
In addition, the image beam 113 reflected by the protective film 144 may form another light spot S2 (indicated by the dotted line) partially overlapping the light spot S1; wherein the overlapping area of the two light spots S1 and S2 may result in a stray light in the projection lens 150. In order to eliminate the stray light, a mask (not shown) configured to block the overlapping area of the two light spots S1 and S2 is disposed in the projection lens 150. As shown in FIG. 4, although the stray light is eliminated; however, another light-loss area is accordingly occurred on the left side of the light spot S1. Thus, the conventional projector 100 has relatively poor light utilization efficiency.